New evidence from a 300-year-old supernova remnant in the Large Magellanic Cloud suggests that some Type Ia supernovae [1]may result from a "double detonation " -- where a helium shell ignites first, triggering a second core explosion in a white dwarf before it reaches critical mass. "While the physics of the process itself are interesting, the key question this raises is whether type Ia supernovae really are all equally bright," writes Ars Technica's John Timmer. "If they can detonate with substantially less mass than is needed for direct ignition of the core, then it's possible that some of them could be considerably less bright." However, the research team notes that additional factors -- such as the influence of binary systems or secondary detonations -- could further complicate the picture. Ars Technica reports:

> "The detonations in the carbon-oxygen core and the helium-rich shell result in qualitatively different yield products," the researchers behind the new work [2]write in a paper describing it . In the paper, they focus on calcium, which there are two ways of producing. One is from the outer shell of helium, via fusion before the detonation dilutes the material. A second batch of calcium is produced through the fusion of the core material as it's ejected in the supernova, which prevents further fusion events from converting it to even heavier elements. (Material deeper in the core does end up getting fused into heavier material.) Because it's produced by both of the detonations, models predict that the expanding sphere of debris will contain two different shells of calcium, with some space in between them. To find evidence for these shells, the researchers checked an older supernova remnant, which allows enough time for the movement of material to separate the shells by enough distance that they can be resolved from Earth.

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> They focused their observations on a supernova remnant named SNR 0509-67.5, located in the nearby Large Magellanic Cloud. SNR 0509-67.5 is estimated to be a bit over 300 years old, meaning material has had enough time to move a significant distance away from the site of the explosion. Imaging using a spectrograph on the Very Large Telescope allowed them to resolve what, in effect, was a spherical sulfur sandwich, with the role of the bread played by calcium. In other words, if you were to travel away from the site of the explosion, you would first hit a layer of ionized calcium, followed by ionized sulfur, and then run into a second layer of ionized calcium. This is exactly what computer models that simulate double detonations predict. So, the researchers suggest it is strong support for that hypothesis. The researchers say that the details suggest that SNR 0509-67.5 was a white dwarf with roughly the same mass as the Sun when it exploded, and that its explosion was likely triggered by the detonation of a helium shell with only three percent of the Sun's mass.



[1] https://arstechnica.com/science/2025/07/new-evidence-that-some-supernovae-may-be-a-double-detonation/

[2] http://dx.doi.org/10.1038/s41550-025-02589-5